1. Field of the Invention
The present invention relates to a brushless motor having plural excitation phases as well as a driving control device for the brushless motor and, more particularly, to a brushless motor suited to a drive source for an electrically-operated power steering system as well as a driving control device for such a brushless motor.
2. Description of the Related Art
Brushless motors used as drive sources for the power steering systems of automobiles are motors having three or more excitation phases, and are driven by means of excitation currents of rectangular waveforms.
For example, in the case of a 5-phase brushless motor, a motor driving circuit rotationally drives its rotor by exciting 5-phase excitation coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d hereinafter referred to also as xe2x80x9ca-phasexe2x80x9d to xe2x80x9ce-phasexe2x80x9d) by a rectangular wave current while switching the coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d sequentially from phase to phase by a 4-phase excitation method of simultaneously exciting four phases, under control of a control circuit such as a microcomputer, the 5-phase excitation coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d being disposed to surround the outer circumferential surface of the rotary element(rotor) of the motor in the state of being spaced apart by an electrical angle of 72 degrees. In the 4-phase excitation method, motor currents flow in four phases from among five phases, and the coil resistances of the respective excitation coils are formed to be all equal so that currents can flow in the respective phases with good balance.
Such a motor driving circuit is normally made of ten field effect transistors(FETs). Among these ten transistors, each pair of two corresponding transistors are connected in series to form five series transistor circuits, and each of the series transistor circuits is connected between the positive and negative terminals of a power source, and the connection between the two transistors of each of the series transistor circuits is connected to each of the five excitation coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d interconnected by a Y-shaped star connection, thereby being connected to the coil circuit of the motor.
The direction and length of an excitation current(rectangular wave) which is supplied to each of the excitation coils from the motor driving circuit are as shown in FIG. 1 by way of example with respect to the rotational angle(electrical angle) of the rotor. Specifically, the excitation coils are switched sequentially from phase to phase by an electrical angle of 36 degrees, thereby exciting one phase coil through an electrical angle of 144 degrees to continuously rotate the rotor. In FIG. 1, letting xcex8 be the electrical angle, (1) to (10) denote respectively the following intervals: 0xc2x0xe2x89xa6xcex8 less than 36xc2x0, 36xc2x0xe2x89xa6xcex8 less than 72xc2x0, 72xc2x0xe2x89xa6xcex8 less than 108xc2x0, 108xc2x0xe2x89xa6xcex8 less than 144xc2x0, 144xc2x0xe2x89xa6xcex8 less than 180xc2x0, 180xc2x0xe2x89xa6xcex8 less than 216xc2x0, 216xc2x0xe2x89xa6xcex8 less than 252xc2x0, 252xc2x0xe2x89xa6xcex8 less than 288xc2x0, 288xc2x0xe2x89xa6xcex8 less than 324xc2x0and 324xc2x0xe2x89xa6xcex8 less than 360xc2x0.
In this example, the a-phase current flows in the plus direction through the intervals (1) and (2), then returns to xe2x80x9c0xe2x80x9d in the interval (3), then flows in the minus direction through the intervals (4) to (7), then returns to xe2x80x9c0xe2x80x9d in the interval (8), and again flows in the plus direction through the intervals (9) and (10) and back in the interval (1). The b-phase current flows in the plus direction through the intervals (1) and (4), then returns to xe2x80x9c0xe2x80x9d in the interval (5), then flows in the minus direction through the intervals (6) to (9), then returns to xe2x80x9c0xe2x80x9d, in the interval (10), and again flows in the plus direction in the interval (1). The c-phase current flows in the minus direction in the interval (1), then returns to xe2x80x9c0xe2x80x9d in the interval (2), then flows in the plus direction through the intervals (3) to (6), then returns to xe2x80x9c0xe2x80x9d in the interval (7), and again flows in the plus direction through the intervals (8) to (10) and back in the interval (1). The d-phase current flows in the minus direction through the intervals (1) to (3), then returns to xe2x80x9c0xe2x80x9d in the interval (4), then flows in the plus direction through the intervals (5) to (8), then returns to xe2x80x9c0xe2x80x9d in the interval (9), and again flows in the plus direction in the interval (10). The e-phase current remains xe2x80x9c0xe2x80x9d in the interval (1), then flows in the minus direction through the intervals (2) to (5), then returns to xe2x80x9c0xe2x80x9d in the interval (6), then flows in the plus direction through the intervals (7) to (10), and again returns to xe2x80x9c0xe2x80x9d in the interval (1). Accordingly, at the boundary between each of the intervals (1) and (10)(at the time of switching performed every 36 degrees in electrical angle), two of the five excitation coils are switched in the mutually opposite directions.
This switching of such an excitation current is in principle represented by the rise or the fall of a rectangular wave as shown in FIG. 1. However, actually, the waveform of the rise or the fall does not change perpendicularly to the horizontal axis and a certain period of time xcex94t(about three times the time constant of the motor circuit) is taken until the excitation current rises in the plus direction or falls in the minus direction.
For example, at the boundary between the intervals (8) and (9) of FIG. 1(288 degrees in electrical angle), the a-phase current rises from xe2x80x9c0xe2x80x9d to a plus constant value, while the d-phase current falls from the plus constant value to xe2x80x9c0xe2x80x9d, and the b-phase current and the c-phase current remain at the minus constant value with the e-phase current remaining at the plus constant value. FIG. 2 shows on an enlarged scale the variations in the waveforms at this boundary.
Specifically, the a-phase rise current gradually increases from xe2x80x9c0xe2x80x9d to the plus constant value during the time xcex94t, while the d-phase fall current decreases from the plus constant value to xe2x80x9c0xe2x80x9d during time xcex94t1 shorter than the time xcex94t (smaller than the time constant of the motor circuit). During this time, the other three phases xe2x80x9cbxe2x80x9d, xe2x80x9ccxe2x80x9d and xe2x80x9cexe2x80x9d remain unswitched. Letting ia, ib, ic, id and ie represent respectively the five phase currents, the relationship of the following expression (1) is established among these currents:
ia+id+ic=xe2x88x92(ib+ic)=Ixe2x80x83xe2x80x83(1) 
Accordingly, as the a- and d-phase currents vary as described above, the b-, c- and e-phase currents also vary. In other words, since the a-phase current and the d-phase current differ in current variation rate, the total value of these two phase currents does not become a steady value and the b- and c-phase currents vary as shown in FIG. 2, so that the e-phase current also varies during the time xcex94t. These current variations cause transient torque variations.
The reason why the current variation rates of two phase currents differ between their rises as well as their falls as described above is as follows.
Let xe2x80x9cVbxe2x80x9d denote a power source voltage to be supplied to the motor driving circuit, and xe2x80x9cVnxe2x80x9d denote a voltage provided at the central connection point of the star-connected excitation coils xe2x80x9caxe2x80x9d to xe2x80x9cexe2x80x9d. In addition, let (1) and (2) in FIG. 2 denote the interval of the time xcex94t1 and the interval of time xcex94t2 (=xcex94txe2x88x92xcex94t1), respectively.
In the interval (1), the d-phase(OFF-phase) current id, which is switched from plus to xe2x80x9c0xe2x80x9d, lowers to zero(0) from half(I/2) of an energization current I supplied to the motor from the motor driving circuit, at a variation rate according to a voltage xe2x88x92Vn, a counter-electromotive voltage Ed of the coil and the time constant of the motor circuit. At this time, letting VOFF denote a voltage to be applied to the OFF-phase equivalent circuit, VOFF=xe2x88x92Vnxe2x88x92Ed less than 0, and Vn approximates Vb/2. On the other hand, the a-phase(ON-phase) current ia, which is switched from xe2x80x9c0xe2x80x9d to plus, rises from zero(0) at a variation rate according to the voltages Vb and xe2x88x92Vn, a counter-electromotive voltage Ea of the coil and the time constant of the motor circuit. At this time, letting xe2x80x9cVONxe2x80x9d denote a voltage to be applied to the ON-phase equivalent circuit, xe2x80x9cVON=Vbxc3x97Duty1(PWM duty)xe2x88x92Vnxe2x88x92Eaxe2x80x9d.
If the current id is explained using an expression, the current id is expressed from the OFF-phase equivalent circuit by the following expression (2):
id(t)=(I/2)exe2x88x92t/T+(VOFF/R)(1xe2x88x92exe2x88x92t/T)xe2x80x83xe2x80x83(2) 
∴when t=0, id=I/2,
where xe2x80x9cTxe2x80x9d denotes the electrical time constant of the equivalent circuit and xe2x80x9cRxe2x80x9d denotes the resistance of the equivalent circuit.
In addition, the current ia is expressed from the ON-phase equivalent circuit by the following expression (3):
ia(t)=(VON/R)(1xe2x88x92exe2x88x92t/T)xe2x80x83xe2x80x83(3) 
∴when t=0, ia=0, and for txe2x86x92∞, ia=VON/R=I/2.
Accordingly, the variation rates of the respective OFF-phase and ON-phase currents id and ia become as follows:                                                                                           ⅆ                                                            i                      d                                        ⁡                                          (                      t                      )                                                                      /                                  ⅆ                  t                                            =                                                                    -                                          (                                              1                        /                        T                                            )                                                        ⁢                                      (                                          I                      /                      2                                        )                                    ⁢                                      ⅇ                                                                  -                        t                                            /                      T                                                                      +                                                      (                                          1                      /                      T                                        )                                    ⁢                                      (                                                                  V                        OFF                                            /                      R                                        )                                    ⁢                                      ⅇ                                                                  -                        t                                            /                      T                                                                                                                                              =                                                -                                      (                                                                  I                        /                        2                                            -                                                                        V                          OFF                                                /                        R                                                              )                                                  ⁢                                  (                                      1                    /                    T                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                                          =                                                -                                      (                                                                  I                        /                        2                                            +                                              Vn                        /                        R                                            +                                              Ed                        /                        R                                                              )                                                  ⁢                                  (                                      1                    /                    T                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                          (        4        )                                                                                                      ⅆ                                                            i                      a                                        ⁡                                          (                      t                      )                                                                      /                                  ⅆ                  t                                            =                                                (                                      1                    /                    T                                    )                                ⁢                                  (                                                            V                      ON                                        /                    R                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                                          =                                                (                                      I                    /                    2                                    )                                ⁢                                  (                                      1                    /                    T                                    )                                ⁢                                  ⅇ                                                            -                      t                                        /                    T                                                                                                          (        5        )            
In the expressions (4) and (5), since (I/2+Vn/R+Ed/R) greater than I/2, the current variation rate of the OFF-phase is greater than the current variation rate of the ON-phase. Particularly in the case where the resistance R of the equivalent circuit is small, or the power source voltage Vb(≈2Vn) is large, or the counter-electromotive voltage Ed is large owing to high-speed rotations, the current variation rate of the OFF-phase is considerably greater than the current variation rate of the ON-phase. Therefore, the time(xcex94t1) required for the OFF-phase current id to lower from I/2 to xe2x80x9c0xe2x80x9d is longer than the time (xcex94t) required for the ON-phase current ia to rise from xe2x80x9c0xe2x80x9d to I/2; that is, the ON-phase current ia does not reach I/2 at the end of the interval (1) and is still rising. After that, in the interval (2), the ON-phase current ia finally reaches the steady value (I/2), but the time xcex94t2(twice to three times the time constant of the motor circuit) is required until that instant. Accordingly, the current variation rates of two switched phase currents differ between their rises as well as their falls.
In the control of excitation currents by the above-described prior art motor driving circuit, since the variation rates of switched currents(for example, the a-phase and the d-phase shown in FIG. 1) differ, the currents of unswitched phases(for example, the b-, c- and e-phases shown in FIG. 1) vary and transient torque variations due to these current variations occur.
To restrain the current variations due to phase switching which cause such torque variations, it is preferable to control each of the phase currents. However, in such control, each of the phase currents needs to be detected, and two or more current detecting circuits are needed. Particularly in the case of a 5-phase brushless motor, because a 4-phase excitation method is adopted, its motor driving circuit needs four current detecting circuits and four current loops, so that there is the problem that the construction of the driving circuit becomes complicated and costs become high.
To solve the problem, it has been proposed to provide, for example, an apparatus for controlling the driving of a brushless motor having plural excitation phases which apparatus includes a driving part which generates an excitation signal to be supplied to each excitation phase of the brushless motor, and a control part which performs ON/OFF switching of, and determines the direction of, the excitation signal for each excitation phase, the control part controlling the variation rates of excitation signals of phases to be switched during the switching. According to this construction, the control part performs ON/OFF switching of, and determines the direction of, the excitation signal to be supplied from the driving part for each excitation phase of the brushless motor. By controlling the variation rates of the excitation signals to be switched during the switching, it is possible to make the current variation rates of the two switched phases coincident with(or similar to) each other. Thus, the current variations of the unswitched phases are restrained, and the above-described torque variations are eliminated.
In addition, since the excitation signals can be generated merely be detecting currents flowing in a motor circuit, it is not necessary to detect each phase current for the purpose of controlling the variation rates of the excitation signals, and a circuit construction for control does not become complicated.
As another apparatus for controlling a current variation rate during commutation, it has been proposed to provide a motor driving control apparatus for driving and controlling a brushless motor having plural excitation phases without using two or more current detecting circuits which detect excitation currents of the brushless motor, which apparatus includes a driving part which generates an excitation signal to be supplied to each excitation phase of the brushless motor, and a control part which performs ON/OFF switching of, and determines the direction of, the excitation signal for each excitation phase, the control part generating the excitation signal so that the total value of excitation currents of the respective excitation phases of the motor during the switching is kept constant. This apparatus is of the type that drives the brushless motor by means of rectangular waves by using a single current detecting circuit, and controls the current variation rates of rise phases and fall phases during phase current switching, thereby keeping constant the motor current during phase switching so that current variations and electromagnetic torque variations can be restrained. Accordingly, the above-described apparatus can realize a high-performance servo motor which is inexpensive and has low current variation and low torque variation.
However, in the above-described method of driving the brushless motor by means of rectangular waves and controlling the variation rates of fall currents during commutation, as the rotational speed of the motor becomes faster, the proportion of commutation transient time in commutation interval time becomes higher. Incidentally, the term xe2x80x9ccommutation interval timexe2x80x9d means the time required from the starting time of a certain commutation until the starting time of the next commutation, and the term xe2x80x9ccommutation transient timexe2x80x9d means the time for which phase currents are in a transient state during commutation operation.
As shown in FIG. 3, if the transient process time during which the rise-phase phase current id gradually lowers exceeds xc2xd of the commutation interval time(the time for which the motor rotates at a constant speed), there occurs the phenomenon that the polarity of the rise-phase counter-electromotive voltage Ed changes and the fall-phase phase current id contrarily rises. In FIG. 3, xe2x80x9ct1xe2x80x9d denotes the starting time of a commutation 1, xe2x80x9cxcex81xe2x80x9d denotes the electrical angle of a rotor position at the time t1, xe2x80x9ct2xe2x80x9d denotes the starting time of the next commutation 2, xe2x80x9cxcex82xe2x80x9d denotes the electrical angle of the rotor position at the time t2, xe2x80x9ct3xe2x80x9d denotes the time at which the polarity of an OFF-phase counter-electromotive voltage changes, and xe2x80x9cxcex83xe2x80x9d denotes the electrical angle of the rotor position at the time t3. In this manner, the rise-phase current rises, whereby current variations, torque variations and noise occur in the motor.
In addition, if the fall-phase current continues to flow for xc2xd or more of the commutation interval time, the polarity of the counter-electromotive voltage of that phase changes, so that in the phase a rotational torque occurs in the opposite direction to the original rotational torque of the motor and the total torque of the motor lowers. The lowering of the motor torque depends on the rotational speed of the motor, and if the motor is to be used as a torque assist device for an electrically-operated power steering system of a vehicle, there is the problem that an operator suffers a viscous steering feeling.
The present invention has been made in view of the above-described problems, and provides a brushless motor and a driving control device therefor both of which can restrain the occurrence of current variations, torque variations and noise in the brushless motor and which can be applied to a torque assist device for an electrically-operated power steering system, thereby constructing an electrically-operated power steering system of low noise and good steering feeling.
The present invention relates to a brushless motor having a current variation rate control part which controls a current variation rate during commutation, and having plural excitation phases, as well as to a driving control device for such brushless motor, and the present invention is achieved in such a way that the current variation rate control part terminates commutation transient time which is the time for which phase currents are in a transient state during a commutation operation, within xc2xd of commutation interval time which is the time required from the starting time of a certain commutation until the starting time of the next commutation.
The present invention is more effectively achieved in such a way that the electrical time constant of the brushless motor that contains the impedance of a driving circuit is made ⅙ or less of the commutation interval time, or in such a way that the current variation rate control part controls the commutation transient time by commutation-phase current control using the rotational speed of the brushless motor as a variable, or in such a way that the current variation rate control part controls the commutation transient time by commutation-phase current control using the rotational electrical angle of the brushless motor as a variable.
Moreover, the present invention is more effectively achieved in such a way that the current variation rate control part limits the interval during which to supply a driving current for a fall phase of the commutation phase of the excitation coil.